A wide variety of processes have been developed for making ultra-high purity glass optical fibers for lightwave transmission systems. In most of these processes, an optical fiber preform is prepared first, and optical fiber is drawn from the preform by heating the preform to a softened state, then pulling a fine strand of glass from the preform. The processes for making the preform rely in general on techniques for preparing very pure glass bodies using vapor phase chemical reactions. Among the most notable are Modified Chemical Vapor Deposition (MCVD), Vapor Axial Deposition (VAD), and Outside Vapor Deposition (OVD). A large number of variants of these basic approaches are known. In all of these, the glass preform body is made by depositing glass soot on a substrate. The soot is formed by an in situ reaction of gaseous glass precursors near or at the substrate. The result is that the deposited glass material has a purity close to that of the precursor materials. As is well-known, the soot—a powdery material—is consolidated into solid glass by sintering the powder particles and coalescing them into solid glass bodies or preforms, which can then be drawn into long lengths of optical fibers.
A limitation of these techniques is that the glass compositions for the preform are limited to constituents that can be conveniently provided in vapor form, and can be reacted with a silica precursor vapor to form doped glass. Another limitation of these techniques is that both the composition and particle size of the soot are indirectly controlled. The composition of the soot is controlled mainly by the flow of precursor gases to the soot torch, and the particle size is determined by several variables including the temperature of the torch. Both of these parameters are subject to process variations. While the thermodynamics of the MCVD process is well understood, and the control means on commercial MCVD apparatus are relatively sophisticated, in principle, the final glass composition is still controlled only indirectly, thus being subject to process variation. Moreover, while a variety of common dopant precursors are available in liquid form, with a reasonably high vapor pressure that allows for vapor phase processing, many potential dopants are not. Common dopants such as GeO2, B2O3, and P2O5, form halides (e.g. GeCl4, POCl3, BCl3) that are well adapted for vapor phase reactions. However, other potentially attractive and useful dopant elements do not form compounds as easily suited for vapor phase processing. Among these are the rare earth elements, notably, Er, Nd, Yb, Sm, La, Ce, Pr, Pm, Gd, Tb, Dy, Ho, Tm, Lu; Group IIIA elements such as Al and Ga; alkali elements such as Li, Rb, Na, K, Cs; transition group elements such as Cr, Fe, Ni, Zn; alkaline earth elements such as Ca, Ba, Sr; Group IVA elements such as Sn and Pb; and Group VA elements such as As, Sb, and Bi. Thus it is evident that there exists a large number of potential glass compositions that are not well adapted to the known vapor phase methods for glass preform manufacture.
Solution doping techniques are also known, and were developed in part to allow these alternative dopants to be incorporated into preform designs. In the typical solution doping methods, a glass soot is first prepared by one of the known vapor phase reaction techniques (e.g., MCVD, VAD) and the soot is soaked in a liquid solution containing a compound of the selected dopant. There are drawbacks to this approach also. A solvent must be found in which the dopant compound is soluble, and moreover which is benign to the final glass product. In addition, the amount of dopant incorporated into the final product is highly sensitive to the nature of the soot, i.e. the particle size, density, porosity, uniformity, and other microstructural features of the soot layer.
Applying particle mixtures directly to the glass surface in the form of a powder slurry has also been suggested. See PCT 01/53223 A1. However, this approach appears not to have been successfully tried, and the technology remains focused on vapor phase pyrolytic methods.